There has so far been known and used a speed change gear comprising a group of clutches comprised of a plurality of speed stage clutches, and forward/reversing clutches, and a group of valves comprised of valves for speed stages for selectively supplying pressurized fluid into these clutches and valves for driving in forward and reverse. A valve for any selected one of speed stages and the valve for forward running or the valve for reversing are changed over so as to put the clutch for the selected speed stage and either the forward running clutch or the reversing clutch, respectively, in engaged condition, thereby changing the speed change gear to any desired speed stage.
This speed change gear is arranged to control speed changes of the vehicle as described below.
Stating in brief, the arrangement is made such that the vehicle speed and the degree of opening of the accelerator, etc. are input to a controller. The controller compares a vehicle speed V.sub.1 set at each speed stage with the actual vehicle speed V.sub.0, and outputs a downshift command in case V.sub.1 is more than V.sub.0 and output an upshift command in case V.sub.1 is less than V.sub.0 so as to disengage or release the presently engaged clutch and then engage a clutch corresponding to the next speed stage, thereby automatically changing the vehicle speed.
Such a method of controlling speed changes is arranged to upshift or downshift the vehicle when conditions for upshifting or downshifting are met. Depending on running conditions of the vehicle, traction load increases or decreases suddenly so that conditions for upshifting and those for downshifting are met repeatedly for a short time thereby causing repetition of upshifts and downshifts for a short time, so-called "shift hunting" which results in unstable control of speed changes.
To solve this problem, it is only necessary to provide, after completion of speed change operations, an appropriate time lag from the time when conditions for upshifting or downshifting are met until an upshifting command or a downshifting command is output.
Stating more specifically, to eliminate the shift hunting, it is only necessary to inhibit the speed change gear from conducting any speed change until a preset time has passed after completion of speed change operations, even though the above-mentioned conditions for upshifting or downshifting are met. This preset time is referred to as speed change inhibiting time.
However, if the speed change inhibiting time is too long, then an extended period of time is needed from the time when conditions for a speed change are met until completion of the speed change. As a result, upon starting and accelerating the vehicle, a longer time is required to upshift (or increase the speed of) the vehicle causing poor acceleration, while if the speed change inhibiting time is kept short, then an improved acceleration is obtained, but the abovementioned shift hunting is liable to occur.
Further, there is known a hydraulically actuated type multiple-stage speed change gear comprising a plurality of hydraulic clutches and gear trains, and arranged such that a speed change of the gears trains to a predetermined speed stage can be conducted by operating or engaging a predetermined clutch.
Such a hydraulically actuated multiple-stage speed change gear is used in combination with a torque converter to increase the tractive force available when the vehicle is started.
Since a large-sized construction vehicle, for example a large-sized dumptruck, is required to run at a high speed, a torque converter fitted with a direct coupled clutch which connects pump impellers and turbine blades thereof is used so as to render the direct coupled clutch on when the vehicle is running to enable a high torque to be transmitted efficiently. However, controls of speed changes are made such that at the time of a speed change the direct coupled clutch is rendered off and the load on the hydraulic clutch of the speed change gear is reduced by the torque converter, and after completion of the speed change the direct coupled clutch is rendered on, or engaged.
More specifically, if a speed change of the gear trains is made with the direct coupled clutch kept on, then a high load is exerted on the hydraulic clutch thus increasing shocks due to the speed change. Therefore, at the time of a speed change, the direct coupled clutch is rendered off to reduce the load exerted on the hydraulic clutch thereby reducing shocks due to the speed change.
According to such a method of controlling speed changes, at the time of speed change both speed change shocks due to engagement of the hydraulic clutch and those due to engagement of the direct coupled clutch will occur so as to increase the magnitude of shocks which occur at the time of the speed change, thus transmitting unpleasant jolts to the driver and passengers.
Furthermore, if the capacity of the hydraulic clutch is increased, then the load exerted on the hydraulic clutch when it is rendered on can be reduced to some degree, however, the speed change gear becomes larger and more expensive than necessary.